In this study, the relationship between the interannual variability of Arctic-midlatitude thermal gradient (AMG) and the winter atmospheric blocking frequency in the Ural region (UBF) is investigated in the ERA5 reanalysis product from 1940 to 2023. In particular, the paper focuses on the large-scale atmospheric circulation patterns associated with high UBF concomitant to weak AMG and vice versa, revisiting the more common and documented relationship connecting intense Ural blocking activity to strong AMG. Results show that displacements of the atmospheric blocking from the Ural region towards the Arctic lead to anomalous southerly thermal advections at polar latitudes and stronger AMG. On the other hand, high blocking frequency co-occurring in the Ural, Greenland and Chukotka regions lead to weaker AMG by limiting northward heat advections towards the Arctic region. These findings highlight a more complex picture of the role of subpolar atmospheric circulation in controlling the AMG.

It is well known that oceanic internal waves (IWs) can be generated by several mechanisms, among which is tidal flow over shallow bathymetry and atmospheric forcing. In recent years, theoreticians have hypothesized that IWS generation can take place also without external forcing involved. This requires unbalanced flow as encountered in the sub-mesoscale regime of the ocean. Thus, sub-mesoscale eddies (SMEs) came into the focus of theoreticians as a source of IWs, but no observational evidence has been presented until now. In this paper, we present, for the first time, observational data that show that ISWs can be generated by SMEs. To this end, we have analyzed 5 high-resolution synthetic aperture radar (SAR) images acquired by the Sentinel-1 satellite in the Adventure Bank area of the Strait of Sicily in the Mediterranean Sea, which show sea surface signatures of ISWs surrounding SMEs. To support our interpretation of the SAR images, we have used as ancillary data high-resolution sea surface temperature (SST) and chlorophyll-a (Chl-a) data from the SLSTR and the OLCI sensors onboard the Sentinel-3 satellites, as well as model hydrology and currents. The ISWs radiate from SMEs formed during the decaying stage of cold filaments, which have their origin in upwelling events along the southern coast of Sicily.